1. Field of the Invention
The present invention relates to a determining device for determining a failure in an engine cylinder of an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 61-258955, discloses a determining device for determining a failure in an engine cylinder of an internal combustion engine having a plurality of cylinders. In this device, an engine speed during a combustion process in each cylinder is sequentially detected, and variations in the engine speed at the successive stages of the combustion process in each cylinder are calculated. Note the variation in engine speed is defined as the difference between two successive engine speeds at two successive stages of the combustion process. The variation .DELTA.NE in the engine speed is calculated from the following equation. EQU .DELTA.NE=NEB-NE
Where NE: an engine speed in a present stage of the combustion process
NEB: an engine speed in a stage of the combustion process immediately preceding the present stage of the combustion process.
When the variation in the engine speed is larger than a predetermined constant reference value X, a misfire count value is increased by 1, and when the misfire count value becomes larger than a predetermined failure count value, a failure has occurred.
In this device, however, a problem arises in that it cannot be exactly determined whether or not a failure has occurred in an engine cylinder, as described below.
FIGS. 9 illustrate the occurrence frequency of the variation .DELTA.NE in the engine speed under a low engine load, and FIG. 10 illustrates the occurrence frequency of the variation .DELTA.NE in the engine speed under a high engine load. Note, No. 1, No. 2, No. 3, and No. 4 cylinders are normal functioning cylinders. Referring to FIGS. 9 and 10, the variation .DELTA.NE in the engine speed in the failed cylinder is increased in accordance with an increase of an engine load, and a distribution area of the variation .DELTA.NE in the engine speed in each normal functioning cylinder is extended in accordance with the increase in the engine load.
Accordingly, when the predetermined reference value X is fixed at X.sub.1, as shown in FIG. 9, it is possible that it is determined that the normal functioning cylinder (for example, No. 3 cylinder) is a failed cylinder, under a high engine load, as shown in FIG. 10.
Conversely, when the predetermined reference value X is fixed at X.sub.2 as shown FIG. 10, a failed cylinder cannot be detected under a low engine load, as shown in FIG. 9.